The matrix detectors that use multiple individual sensors deliver a video signal representative of the observed scene. This video signal is formed by the transfer and multiplexing of charges released by the sensors according to the lighting received by each sensor, using known electronic devices of the charged coupled or charge transfer type. The quantity of charges transferred is also a function of the charge integration time. In IR imaging, the detector and the charge transfer device are arranged in a cryostatic chamber cooled by systems operating according to known techniques.
As a general rule, an image comprises a scene (structured) on a uniform (and therefore unstructured) background and the video signal then includes a continuous component that is a function of the background luminescence and a variable component representing the scene. The continuous component generally has a relatively high value compared to the variable component representing the scene. For example, in the 3 to 5 μm or 8 to 12 μm bands, a temperature difference between the scene and the background equal to one degree Celsius is typically reflected in a variation by a few % of the video signal relative to the continuous component.
The imaging devices that use matrix detectors and more particularly the IR imaging devices are subject to the following constraints:                on the one hand, the contrast of the objects is low: for a temperature range of 1° C., it is, as has just been shown, a few % whereas IR imaging involves temperature differences of the order of 1/10° C.;        on the other hand, the various individual sensors of a matrix detector do not generally have the same response, and these responses are not perfectly stable in time: in practice, during the analysis of a uniform background, the scattered responses from the sensors reproduce the intrinsic variations of the sensors and constitute a signal that is unstable in time including noise overlaid on a continuous component equal to the average response;        finally, the structural flux of the imaging device seen by each sensor, whether direct (through emissivity of the optical “duct”) or whether seen by stray reflection on the diopters of the optical combination (Narcissus effect), varies with the fluctuations in space and time of the internal temperature of the camera. This stray flux is overlaid on the useful flux and thus falsifies the perception of the scene.        
Consider a matrix detector which comprises sensors (i, j) distributed on I rows and J columns, with 1≦i≦I, 1≦j≦J. It will be recalled that, as a first approximation, the output Yij of each sensor (i, j) of the IR matrix detector is linear according to the flux F received (sum of the scene flux and of the structural flux): Yij=Oij+Gij×F. The term Oij, commonly called “offset”, represents the dark current of the individual sensor and Gij represents the gain of the sensor.
Generally, a precalibration is performed in the factory, by placing the equipment facing a uniformly black body and by varying the temperature thereof, this making it possible to calculate for each individual sensor, the gain and “offset” corrections with which to reconstitute a perfectly uniform image. These correction tables take account of the defects specific to the detector and the non-uniformities of the structural flux in the calibration conditions, and become irrelevant as soon as the temperature conditions in the equipment deviate too far from the calibration conditions. It will be understood that, in the final analysis, corrections must regularly be made to the “offsets” of the individual sensors during the operational use of the equipment.
One effective correction principle consists in periodically replacing the flux from the scene—essentially structured—with a reference flux that is spatially unstructured (ideally uniform) and representative of the average level in the scene; in these conditions, it is possible to measure the stray variations of the signal due to the detector and to the structural flux, and therefore to restore, after subtraction, a faithful image of the scene.
One conventional calibration technique consists in presenting to the detector the light flux from a black body whose temperature is adjusted to the average temperature of the observed scene using a servo control loop; the black body is placed in the optical path using a dedicated opto-mechanical switch, for example a tilting mirror. This temperature-locked black body system is complicated, in particular when its temperature is much colder than that of the camera: this poses numerous problems associated with condensation on the black body, the speed of response of the servo control loop, the control and differential measurement of the temperature, etc. Also, to guarantee a quality calibration, the black body must be raised to a precise temperature, and when the black body cannot be placed in the immediate vicinity of a pupil plane, it is essential to eliminate the thermal gradients along the emitting surface, the emissivity of which must be known and controlled.
Similarly, the use of a shutter blocking the optical path makes it possible to perform a calibration function with reduced performance levels but freed of the constraint of integration of a reference black body.
Another device is described in the patent FR 92 14307, which relates to single-focus (i.e. single-field) IR imaging optical systems: this time, by means of an additional device, a group of lenses dedicated to calibration is inserted or translated into the optical path, which makes it possible to reject the aperture area in the plane of the scene (i.e. to infinity), so as to totally defocus the scene flux while retaining the same field of view of the camera.
In all these cases, in order to calibrate an IR camera, an additional mechanism dedicated to the function is used, which increases the cost, the bulk and the weight of the equipment.
Other calibration techniques are proposed, but at the price of very detrimental operational constraints. Thus, to switch to calibration mode, some manufacturers advise the user to target a very near scene, such as, for example, the ground vertically beneath the imaging device, the camera being focused to infinity; some even recommend the use of an opaque cover blocking the head optic, assumed adjacent to the aperture area, in order to present to each pixel an defocused flux but one whose temperature is not necessarily close to the average scene temperature.
In these last two cases, it will be noted that, during calibration, the user de facto loses his imaging line of sight, which is unsatisfactory from an operational point of view.
The aim of the invention is to obtain a calibrated multifocal IR imaging device that does not include any excess cost or detrimental operational constraints by virtue of the calibration.